Projection lens systems (also referred to herein as "projection systems") are used to form an image of an object on a viewing screen. The basic structure of such a system is shown in FIG. 4, wherein 10 is a light source (e.g., a tungsten-halogen lamp), 12 is illumination optics which forms an image of the light source (hereinafter referred to as the "output" of the illumination system), 14 is the object which is to be projected (e.g., a matrix of on and off pixels of a LCD panel), and 13 is a projection lens, composed of multiple lens elements, which forms an enlarged image of object 14 on viewing screen 16.
Projection lens systems in which the object is a LCD or other pixelized panel are used in a variety of applications, including data display systems. Such projection lens systems preferably employ a single projection lens which forms an image of either a single panel having, for example, red, green, and blue pixels, or three individual panels, one for each color. For ease of reference, the following discussion will be in terms of a projection lens system that employs a single LCD panel, it being understood that the invention can also be used in systems which employ multiple panels and/or other types of pixelization.
There exists a need for a projection lens for use with a LCD panel which simultaneously has at least the following properties: (1) a long focal length, e.g., a focal length greater than the largest dimension of the LCD panel for a large LCD panel, e.g., a LCD panel having a largest dimension greater than about 10 centimeters; (2) a relatively long back focal length, e.g., a back focal length greater than the lens' focal length; (3) a high level of color correction; (4) low distortion; (5) the ability to operate at various magnifications while maintaining an efficient coupling to the output of the illumination system and a high level of aberration correction; (6) low sensitivity to temperature changes; and (7) a minimal number of lens elements, a majority of which are made of plastic, so as to minimize the overall cost of the system.
A long focal length is desired since it permits the use of large LCD panels, e.g., panels having a width greater than 10 cm, such as those having a width on the order of 20 cm, while still maintaining the field angle of the projection lens in a manageable range, e.g., in the range of about 50.degree.-60.degree. (full field). Large LCD panels are desirable since they provide either greater resolution if small pixels are used or easier manufacture if larger pixels are used.
A relatively long back focal length, i.e., the distance from the last lens surface to the LCD panel, is desirable since it allows the output of the illumination system to be in the vicinity of the projection lens for output distances which are relatively large. Relatively large output distances are desirable since they provide relatively shallow entrance angles for the light at the LCD panel.
A high level of color correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. These problems are typically most severe at the edges of the field. In general terms, the color correction, as measured at the LCD panel, should be better than about half a pixel to avoid these problems, e.g., the color correction should be preferably better than about 100 microns for pixels having a characteristic dimension of about 200 microns.
All of the chromatic aberrations of the system need to be addressed, with lateral color, chromatic variation of coma, and chromatic aberration of astigmatism typically being most challenging. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing cathode ray tubes (CRTs) a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction is thus needed from the projection lens.
It should be noted that color aberrations become more difficult to correct as the focal length of the projection lens increases. Thus, the first two criteria discussed above, i.e., a long focal length and a high level of color correction, work against one another in arriving at a suitable lens design.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center. Moreover, projection lenses are often used with offset LCD panels so that the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but increases monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
A projection lens which can efficiently operate at various magnifications is desirable since it allows the projection system to be used with screens of different sizes without the need to change any of the components of the system. Only the object and image conjugates need to be changed which can be readily accomplished by moving the lens relative to the LCD panel. The challenge, of course, is to provide efficient coupling to the output of the illumination system and a high level of aberration correction throughout the operative range of magnifications.
In order to produce an image of sufficient brightness, a substantial amount of light must pass through the projection lens. As a result, a significant temperature difference normally exists between room temperature and the lens' operating temperature. In addition, the lens needs to be able to operate under a variety of environmental conditions. For example, projection lens systems are often mounted to the ceiling of room, which may comprise the roof of a building where the ambient temperature can be substantially above 40.degree. C. To address these effects, a projection lens whose optical properties are relatively insensitivity to temperature changes is needed.
One way to address the temperature sensitivity problem is to use lens elements composed of glass. Compared to plastic, the radii of curvature and the index of refraction of a glass element generally change less than those of a plastic element. However, glass elements are generally more expensive than plastic elements, especially if aspherical surfaces are needed for aberration control. Similarly, a lens design composed of a small number of lens elements is desirable since, like the use of plastic, less elements means less overall cost for the lens system.
The projection lenses described below achieve all of the above requirements and can be successfully used in producing low cost projection lens systems capable of forming a high quality color image of a pixelized panel on a viewing screen.